1. Field of the Invention
The subject invention relates to vehicle maintenance and, more particularly, to an apparatus for withdrawing and replacing transmission fluid in an automatic transmission.
2. Description of the Prior Art
Automatic transmissions require transmission fluid both to operate the driving means within the transmission and to lubricate the internal components within the transmission. Simply draining an automatic transmission and refilling the transmission does not provide adequate exchange of fluid due to the fact that as much as 2/3 of the used transmission fluid can remain trapped in the bell housing of the transmission. Such a procedure can, therefore, only dilute the used transmission fluid with about 1/3 new fluid. Most automatic transmissions employ external cooling systems wherein an internal transmission pump pumps transmission fluid out of the transmission, through a heat exchanger and back to the transmission. By intercepting such an external cooling system, an effective exchange of substantially all of the transmission fluid in a transmission can be accomplished. Several devices have been developed to collect used transmission fluid flowing through a cooling system while replacing such used fluid with new transmission fluid.
Devices for replacing transmission fluid by intercepting the transmission cooling circuit have been known for some time. An example of such a device exchanges transmission fluid by disconnecting a transmission fluid cooling line and using compressed air to force new transmission fluid into the transmission at the same rate at which used fluid is pumped from the transmission by the transmission pump. The rate of new fluid input is controlled by adjusting the pressure of the air applied to the new fluid and the rate at which fluid is expelled is determined by the speed of the transmission pump. While such a system can effectively replace all of the fluid in the transmission, the manual adjustment of air pressure can prove difficult, leading to an over or under filled transmission, potentially damaging the transmission.
Another method which has been employed to exchange transmission fluid involves use of a chamber divided by a piston or bladder, one side of which is filled with new transmission fluid. Used transmission fluid is forced, under pressure of the internal transmission pump, into the chamber on the other side of the piston. The used fluid forced into the chamber forces the new fluid out of the chamber to the transmission. The operating pressure of the transmission must overcome the preloaded pressure of the new fluid in the chamber, which is generally about 30 pounds per square inch (psi). In addition, an excessive pressure, generally over 50 psi, forced into the chamber from the transmission will cause the used fluid to bypass to a reserve reservoir. Therefore, there exists the potential, when using such a device, that the transmission will be filled with an incorrect amount of new fluid.
Other devices have been developed which pump new transmission fluid into the transmission through the transmission cooling circuit while used transmission fluid is removed. The rates of input and output are controlled by manually adjusting valves to match the flow of new transmission fluid with the flow of used transmission fluid. Examples of such a device are disclosed in U.S. Pat. Nos. 5,743,357; 5,535,849; 5,370,160 and 5,626,170, all assigned to the assignee of the present application. While devices of this type have enjoyed considerable commercial success, the fact remains that, for successful operation, some degree of skill is required on behalf of the operator and continuous monitoring is necessary to assure a complete fluid exchange with appropriate filling of the transmission.
In an attempt to automate the fluid exchange process, devices have been developed to monitor the flow rates into and out of the transmission and restrict or increase flow to automatically achieve approximately equal flow rates. While such systems are intended to greatly simplify the procedure for the service technician, such systems typically rely on flow measurement and fail to provide an accurate monitoring of quantity of transmission fluid added to and removed from the transmission as would ideally be desired. The inaccuracy inherent in measuring the flow of transmission fluid is due in large part to the nature of transmission fluid. Fluid viscosity significantly affects the performance of flow meters. The viscosity of transmission fluid is temperature dependant, the rendering traditional flow measurement inaccurate, leading to flow rates which vary with season or time of day or with the operating temperature of the transmission being serviced. Additionally, air bubbles entrained within the transmission fluid will further render such flow measurements inaccurate, due to the fact that traditional flow meters, which cannot distinguish the air bubbles from the transmission fluid, will interpret the volume occupied by the air bubbles as being transmission fluid, causing the flow meter to register a higher than actual flow oftransmission fluid.
Since the nature of the fluid renders conventional flow measurement inherently inaccurate, a method of accurately measuring the actual quantity of fluid exchanged, regardless of the viscosity of the fluid or the amount of air entrained within the fluid, is needed. However, the same problems which plague flow measurement also arise with many methods ofmeasuring volume. For instance, air bubbles seem to increase the overall volume of the fluid thus rendering inaccurate many volume sensors such as floats measuring the volume of transmission fluid removed from the transmission. Also, many devices which measure the height of a fluid can be rendered inaccurate when used on a surface which is not level, as is the case with many shop floors.
Thus there remains a need for a transmission fluid exchange device to automatically replace used transmission fluid expelled from a transmission with an accurate equivalent of new transmission fluid. Such a device would closely monitor the quantity of fluid exchanged, regardless of the viscosity of the fluid or amount of air entrapped therein. It would also be advantageous if the quantities of new and used fluid in the new and used fluid receptacles were sensed before transfer is commenced so it could be determined whether there is sufficient new fluid to complete the transfer and sufficient capacity in the used fluid receptacle. Additionally such a device would desirably employ a measurement method providing data which could easily be electronically processed while avoiding the use of complicated mechanical mechanisms in order to ensure reliability.